Thumb sucker??

Baby K is 4 months old today and she has been trying really hard the last couple of days to get her thumb in her mouth to suck on. She is finally starting to succeed at a more consistent rate lately (even though it looks really awkward). Try as we might she still has not been able to catch on to pacifier sucking for some reason. She will still gag herself on them and get really frustrated because she gets no food from it.